Prismatic lighting fixture



w. E. THOMAS 3,009,054

PRISMATIC LIGHTING FIXTURE Nov. 14, 1961 I INVENTOR. M ////2=?/77 E. Thomas Nov. 14, 1961 w. E. THOMAS 3,009,054

PRISMATIC LIGHTING FIXTURE Filed June 21, 1957 0 2 Sheets-Sheet 2 F'1E'| E IN VEN TOR. Wi/l/am E Thu/27a;

ATTORNEYS United States Patent ()fice 3,009,054 Patented Nov. 14, 1961 3,009,054 PRISMATIC LIGHTING FDCTURE William E. Thomas, San Francisco, Calif assignor to Prism Signs, Inc., San Mateo, Calif., a corporation of California Filed June 21, 1957, Ser. No. 667,108 1 Claim. (Cl. 240-5111) This invention relates generally to light fixtures of the type suitable for general illuminating purposes, and which employ a line type of light source, such as rectilinear fluorescent orfilament lamps.

In the past various types of light fixtures have been employed to obtain desired light distribution patterns. Where one or more overhead line light sources have been employed, it has been common to provide reflectors of the parabolic type disposed behind thelight source, for more eflicient light utilization. Such an arrangement has a number of inherent disadvantages. Particularly when the fixture is viewed from below at an angle, it appears extremely bright in a localized portion of the reflector, and the remainder of the reflector is unilluminated. In addition a parabolic reflector within the aspect ratio of 1 to 3 (which is obtainable with the present invention) provides a nonuniform light pattern. Particularly the light pattern is such that very intense illumination is provided at the center of the pattern, and very little at the edges. Diffusing means may be used with such fixtures, as for example, a screen of translucent material extending in front of the light sources, or diffusing reflectors. Although such expedients may minimize the disadvantages just mentioned, the net result is to spread the light over a relatively wide angle, and thus reduce the usable light output. In other words, in the working area to be illuminated, the amount of light per unit area is reduced.

In general it is an object of the present invention to provide a light fixture which makes use of a line light source, and which makes possible a greater usable light efliciency than has been heretofore possible.

Another object of the invention is to provide a fixture of the above character having low surface brightness, with resulting case on the eyes of an observer looking directly at the same.

Another object of the invention is to provide a light fixture of the above character having an optimum light distribution pattern. More particularly the light is controlled in such a manner that substantially all of the light output from the source is directed downwardly within a limited angle, thus resulting in bright illumination directly below the fixture and providing a minimum amount of light beyond the desired distribution pattern.

Another object of the invention is to provide a fixture of the above character having relatively small housing depth, thereby providing an attractive appearance and facilitating installation.

Another object of the invention is to provide a fixture of the above character which'provides relatively uniform distribution of light over a desired area.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in detail'in conjunction with the accompanying drawing.

Referring to the drawing:

FIGURE 1 is a side elevational view illustrating a light fixture incorporating the present invention.

FIGURE 2 is a cross-sectional detail on an enlarged scale, taken along the line 2-2 of FIGURE 1.

FIGURE 3 is a view like FIGURE 2 but showing the lamp images.

FIGURE 4 is an enlarged cross-sectional detail illustrating the contouring for the lens segments.

FIGURES 5 and 6 are light distribution patterns for one particular example.

The particular fixture illustrated in FIGURES 1 and 2 is intended for overhead installation. It consists of a relatively flat housing 10 adapted to be supported from a ceiling either by directly attaching the housing, or by means of suspension members 11. The front side of the housing is covered by the lens 12, which is formed of a suitable light transmitting material, such as a transparent or translucent plastic. Within the housing there is a light source in the form of a fluorescent tube 13, which is carried at its ends by conventional sockets 14. Note that the tube is located coincident with the vertical center plane of the fixture. A conventional transformer or ballast device is indicated at 15.

Above the tube 13 I provide the curved reflectors 16, which extend the length of the fixture, and which are on opposite sides of the vertical center plane. On the two sides of the tube 13, I provide the flat inclined reflectors 17. Below the tube 13, and between the tube and the lens 12, I provide the curved reflectors 18 which likewise extend the length of the fixture, and which also are disposed on opposite sides of the vertical center plane. As shown particularly in FIGURE 2, light from the sides of the tube 13 may, for example, travel along paths 21 and 22 to the flat reflectors 17, and then be reflected downwardly to the lens 12, along the lines 23 and 24. Curved reflectors 16 receive the light from the upper side of the tube 13, and reflect such light toward the lens 12. Curved reflectors 18 reflect light from the lower side of the tube, and direct it upwardly and outwardly to the curved reflectors 16 and fiat reflectors 17.

The lens 12 is of the Fresnel type and is formed in two sections 12a and 12b, which adjoin along a line coincident with the center vertical plane of the fixture. Each section extends for the length of the fixture. FIG- URE 4 illustrates one lens section, namely the section 12a. Note that it consists of a plurality of lens elements extending the length of the fixture, and which are designed to collect light from the reflector system, and direct it downwardly in a predetermined distribution pattern. In FIGURE 4 the representative marginal lens elements for section 12a are designated as 1a, a representative element which is about midway between the margins as 3a, and representative elements intermediate 1a and 3a, as 2a. Referring particularly to the element 3a, its upper surface 26 is cylindrical, and its lower surface 27 is slightly convex and parallel to the general plane of the lens. The surface 27 can extend beneath several upper surfaces 26 to form in effect a group of elements of similar optical characteristics. Each element 2a has an upper curved surface 28 and a lower planar surface 29. However, in this instance the curvature of surface 28 is distorted from cylindrical as illustrated, whereby the convexity is progressively greater toward that edge of the element nearest the element 3a. Furthermore, the plane of surface 29 is tilted at an acute angle with respect to the plane of the lens. With respect to each element 1a, the upper surface 31 is likewise distorted as for the element 2a, but the distortion is somewhat greater. The lower surface 32 is also tilted with respect to the plane of the lens, and the angle of tilting is greater. As is evident from FIGURES 2 and 3, the distortion of the upper curved surfaces of all the lens elements for each section becomes progressively greater toward th side edges of the same. Likewise the lower surfaces of the elements are tilted at progressively increasing angles.

The fiat reflectors 17 are so positioned whereby the optical effect is to provide two virtual lamp light images 13a and 13b, which fall outside of the housing. Also the arrangement is such that these images are disposed immediately above the centers of the two lens sections 12a and 12b, and are spaced from the lens a distance substantially twice the actual spacing between lamp 13 and the lens. All light rays from one of the images 13a and 13b are essentially downwardly and are directed upon the respective lens sections 12a and 12b. Since the cosine of angle 36 is so nearly unity, there is no discernible difference between the intensity of the light falling upon different portions of each lens section, from each of the virtual images. In other words, as viewed from the exterior, the lens is of substantially the same brightness over its entire area.

As previously explained, the curved reflectors 16 receive light coming from the upper side of the lamp 13, and direct it downwardly upon the lens. Therefore these reflectors conserve light from the lamp which would otherwise not strike the flat reflectors 17. Similarly the reflectors 18 receive light from the lower side of the lamp, and reflect such light to the flat reflectors 17 The reflectors 18 are so dimensioned that they do not interfere with proper uniform illumination of the lens sections. In FIGURE 3 the images of the reflectors 16 are indicated at 16a and 16b, and similarly the images of reflectors 13 are indicated at 18a and 18b.

The paths of light through three representative elements of one lens section are shown in FIGURE 4. For each element 1a, the middle ray 41 is reflected straight downwardly, or at right angles to the plane of the lens 12, while the ray 42 is refracted at a slightly greater angle, and the rays 43 at a slightly lesser angle than the center ray 41. Thus emerging rays corresponding to the rays 42 and 43, are at a small angle 44 with respect to the vertical. The angles 44 are equal by virtue of the distorted curvature of the surface 31. This shaping is dependent upon the index of refraction of the transparent material used for the lens sheet, and the spread of light desired from the lens. Light rays are refracted by lens elements 2a in substantially tthe same manner as previously described. The same applied to lens elements corresponding to element 3a, which have a cylindrical upper surface 26.

FIGURES 5 and 6 illustrate typical light patterns which have been obtained by use of my fixture. FIG- URE 5 is a distribution pattern and taken in a vertical plane crosswise of the fixture. FIGURE 6 is a distribution pattern taken in a vertical plane extending through the longitudinal center of the fixture. Curve 46 represents regions of equal illumination from the fixture, and corresponds generally to the boundaries of the illuminated space. Note that the illumination is sharply confined and concentrated in the space below the fixture. Assuming the illumination of a work area below the fixture, such illumination is relatively uniform within the boundaries of the distribution pattern, and if an observer looks directly at the lens, no bright spots will be noted. The distribution pattern shown in FIGURE 6 was plotted in a similar manner and likewise shows the way in which the light from the fixture is confined and concentrated in the space below the same. The spread of light from the ends of the fixture is somewhat greater than from the sides, because the lens elements extend longitudinally.

It will be evident from the foregoing that I have provided a fixture which attains all of the objects previously stated. Particularly, it provides relatively high lighting efiiciency in that a high percentage of the light from the lamp is effectively utilized. Because of the manner in which the light is concentrated downwardly from the lens, an ordinary observer of the fixture will notice very little light coming from the lens, unless he is within the light distribution pattern. In general the fixture as a relatively large illuminating area, but at the same time it is relatively compact with respect to its vertical dimensioning. In other words, the invention makes possible a housing 10, which is relatively fiat, and which therefore has an attractive appearance, and can be readily installed on a ceiling. A further feature of the invention is that it permits the use of a single light tube or a lamp, while at the same time providing illumination from a relatively large area.

Fixtures in accordance with the present invention can be utilized separately where concentrated lighting is desired, or may be used in multiples, mounted side by side or in any arrangement desired, to illuminate a larger area.

I claim:

In a light fixture, characterized by relative shallowness and reduced surface brightness, a single rectilinear lamp, a housing, said housing enclosing the lamp and having an open side, a planar lens of light transmitting material carried by the housing and extending across said open side, said lens being formed in two duplicate sections joined along the longitudinal center plane of the fixture and having a plurality of longitudinally extending lens elements parallel to said rectilinear lamp, a first pair of reflectors supported by the housing and including flat portions disposed at equal angles on opposite sides of the lamp and extending longitudinally of the fixture, said reflectors also including curved portions extending inwardly to a position on said central plane adjacent a side of the lamp remote from the lens, a second pair of reflectors interposed between the lamp and the lens and having curved portions extending outwardly from a position on said central plane, said last named reflectors directing light at said first pair of reflectors to form two separate spaced apart images capable of indirectly lighting each of said two duplicate lens sections, said images being spaced from said lens sections a distance substantially twice the actual spacing between the lamp and the lens, the lens elements having flat exterior surfaces and curved convex interior surfaces, the curvature for most of the interior surfaces being distort ed from cylindrical with progressively greater distortion toward the edges of the lens sections, the lens elements of each section acting to concentrate indirect light into a relatively narrow pattern extending outwardly from a longitudinal center plane of a section, whereby a light fixture of exceptionally low, uniform surface brightness and providing a concentrated, uniform light distribution pattern is made possible.

References Cited in the file of this patent UNITED STATES PATENTS 1,900,551 Guth Mar. 7, 1933 1,923,653 Ambrose et a1. Aug. 22, 1933 2,125,635 Hobart Aug. 2, 1938 2,318,716 Rolph May 11, 19.43 2,368,014 Franck Ian. 23, 1945 OTHER REFERENCES Architexts, vol. 2, No. 2, May 1941. 

